Observations from Polar during a substorm on April 22, 1998, are used to specify electrodynamic characteristics of the high-latitude auroral boundary on the nightside. Polar was moving equatorward near invariant latitude 72¿, 2305 magnetic local time as it crossed the auroral boundary near the end of the substorm's expansion phase. This boundary was marked by severe east-west plasma flow shears, a reversal of the in-track electric field component, and multiple field-aligned currents. Harmonizing ground measurements with auroral images and in situ particle and field data from Polar reveals five electrodynamic features of the boundary. (1) A 20-min delay occurred between substorm onset and when the total magnetic flux in the polar cap began to decrease. This represents the time that elapsed before reconnection of open lobe flux began along a near-Earth X-line. (2) The reconnection electric field at the ionospheric projection of the X-line ranged between 20 and 70 mV m-1. Reconnection was intermittent, turning on and off at different locations. (3) Electric and magnetic field structures observed by Polar suggest that Alfv¿n waves propagating along the auroral boundary carried a double-layer current. Downward Poynting flux was observed at the poleward auroral boundary associated with these currents. (4) Magnetic and electric field oscillations with periods of ~90 s were detected on open field lines beginning ~4 min before Polar entered the auroral oval. Oscillations with similar frequencies were observed both on the ground near Polar's magnetic footprint and at geosynchronous orbit. This indicates that the oscillations represent a large-scale phenomenon occurring over a large portion of the nightside magnetosphere. Coupling on open field lines derives from fringing fields associated with ionospheric closure of DP 1 currents. (5) Upward flowing hydrogen and oxygen ions were detected at and equatorward of the auroral boundary. Perpendicularly accelerated O+ ions detected in the immediate vicinity of the boundary can be explained by direct acceleration by the ambient electric field perpendicular to the local magnetic field. Equatorward of the boundary, O+ distributions were typical of ion conics. ¿ 2001 American Geophysical Union